1. Field of the Invention
The present invention relates to an inertial sensor that measures angular velocities about three axes and accelerations along the three axes and relates to an electrical or electronic device including the inertial sensor.
2. Description of the Related Art
In automobile and mechanical industries, there is a growing demand for sensors capable of accurately measuring a physical quantity, such as an acceleration or an angular velocity. In general, an object that moves freely in a three-dimensional space is acted upon by an acceleration in any direction and an angular velocity in any rotation direction. To accurately grasp the motion of the object, it is therefore necessary to measure an acceleration along each coordinate axis of an XYZ three-dimensional coordinate system and an angular velocity about each coordinate axis. Consequently, there is an increasing demand for multidimensional acceleration sensors and multiaxial angular velocity sensors having a compact size and high accuracy with low manufacturing cost.
Various multidimensional acceleration sensors have been proposed. For example, PCT Publication Nos. WO 88/08522, WO 91/10118, and WO 92/17759 disclose acceleration sensors for measuring an applied acceleration along each coordinate axis. The features of those acceleration sensors are as follows: Resistors, capacitors, and piezoelectric elements are arranged in predetermined positions in a flexible substrate of each sensor. An applied acceleration is measured on the basis of a change in resistance of each resistor, a change in capacitance of each capacitor, and a change in voltage generated by each piezoelectric element. A weight is arranged on the flexible substrate. When an acceleration acts on the sensor, a force is applied to the weight, so that the flexible substrate is deformed. The deformation is detected on the basis of a change in resistance, a change in capacitance, and a change in generated voltage described above, thus obtaining an acceleration component along each axis.
Multiaxial angular velocity sensors have rarely been reported in literatures and have not been put into practical use. Typical angular velocity sensors are used to measure an angular velocity about a power shaft of a vehicle. Accordingly, those angular velocity sensors each have a function of measuring an angular velocity about a specific axis. A one-dimensional angular velocity sensor is sufficient to obtain a rotational speed of such a power shaft. To obtain an angular velocity of an object that moves freely in a three-dimensional space, it is necessary to individually measure angular velocities about X, Y, and Z axes (hereinafter, also called the three axes) in the XYZ three-dimensional coordinate system. To measure an angular velocity about each of the three axes using a related-art one-dimensional angular velocity sensor, it is necessary to arrange the three related-art sensors in specific directions so that the sensors measure angular velocities about the three axes, respectively. Accordingly, the entire structure of an apparatus for obtaining angular velocities about the three axes is complicated and the manufacturing cost of the apparatus is expensive.
For example, Japanese Unexamined Patent Application Publication No. 2004-294450 discloses an angular velocity sensor capable of measuring angular velocities about the three axes. The sensor employs a method of moving a weight in a circle to measure angular velocities about the three axes, namely, two directions perpendicular to the tangent of the circle and one direction along the axis. To allow the weight to stably move in the circle in the angular velocity sensor, it is necessary to control a variation in 2-DOF structure and electrically control the sensor. However, those controls are difficult. Specifically, it is technically difficult to suppress noise caused by centrifugal separation and a disturbance acceleration. Disadvantageously, it is difficult to obtain the S/N ratio equivalent to that of a related-art angular velocity sensor.
Uniaxial and biaxial angular velocity sensors using microelectromechanical systems (MEMS) technology have been put into practical use. Two or three of those sensors can be used to achieve three-axial angular velocity measurement. For example, it is assumed that a three-axial acceleration sensor and a plurality of uniaxial angular velocity sensors are arranged to construct a three-axial angular velocity detecting apparatus. Disadvantageously, the apparatus has a size of several cm3 or more in consideration of the structures of those sensors. Furthermore, it is technically difficult to simultaneously provide the directions of the X and Y axes and the direction of the Z axis. Accordingly, such a three-axial angular velocity detecting apparatus has not been realized.